This invention relates generally to rotary shear-type shredders, and more particularly to the configuration and arrangement of cutter discs therefor.
Rotary shear-type shredding devices of the type disclosed incorporate first and second parallel horizontal side-by-side counterrotating shafts mounted in a housing. Circular disc-type cutters are stacked on each shaft with circular spacers interposed between each such cutter disc; each cutter disc typically having one hook attached thereto. Usually the hooks curve or slant toward the direction of rotation of the disc and have a backside tangential to the circumference of the cutter. One typical example of the above is U.S. Pat. No. 4,034,918 disclosing a shredding apparatus driven by a hydraulic motor. Each cutter disc employs one slanted, curved hook to grab and pull gravity fed material into and through the cutters.
One problem with this arrangement, however, is that flat or sheet materials are difficult for the hooks to grab. These difficult-to-grab materials include pallets, plywood, press board, cardboard, etc., as well as rubber tires, sheet metal, and scrap. Accordingly, this type of shredder requires additional means to force such materials through the cutters. As such, some manufacturers include a ram-type assist mechanism that pushes difficult-to-grab materials downward into the cutting chamber. Moreover, manufacturers frequently recommend larger feed openings as an additional measure to further alleviate this problem.
Another problem arises with this arrangement because these machines are designed to reverse briefly in case a jamming condition occurs. Ideally, reversal will rearrange the material being shredded so that it can be shredded more easily when the shredder returns to normal operation. Slanted hooks, however, are only marginally effective in rearranging materials upon reversal. The slanted or tangential rear sides of the hooks slide under the material without grabbing it. Other rotary shear-type shredders are discussed in U.S. Pat. Nos. 4,833,866; 4,793,561; 4,702,422; 4,609,155; 4,560,110; 4,034,918; 3,868,062; 3,845,907; 3,664,592; 3,630,460; 3,146,960.
Another apparatus used to break down the solids found in sewage and sludge is a grinder manufactured by Disposable Waste Systems, Inc. Like a rotary shear-type shredder, this apparatus employs parallel grinder shafts with stacked cutter discs on each shaft. A plurality of block-shaped cutter elements are attached to each cutter disc. Each cutter element has a parallelogram shaped cross-section with two opposite leading edges that can cut in each direction. With this arrangement, however, the primary function of the blocks is to cut and grind, not to grab and pull. Thus, although this arrangement is effective for grinding solids found in sewage, and certain dry solids, it is limited to those applications where difficult-to-grab materials are not processed. Lacking any hook-type teeth and having many cutter elements on each disc, this machine is likely to be ineffective in shredding sheet metal or other flat materials.
Accordingly, there is a need for a configuration and arrangement of cutter discs that improve the efficiency of rotary-type shredders to process difficult-to-grab materials.